Apparatus for coating fluorescent lamp tubing



June 23, 1942. E. T. CASELLINI 2,

APPARATUS FOR COATING FLUORESCENT LAMP TUBING Filed Oct. 5, 1940 aSheets-Sheet 1 x1 Ell /ILL. 14

inventor.

Ezio Thomds Casellini Aiforneg.

June 23, 1942.

E. T. CASELLlNl APPARATUS FOR COATING FLUORESCENT LAMP TUBING Filed001;. 5, 1940 3 Sheets-Sheet 2 '5 Ii 1" J*3 BY M M,

ATTozA/EY June 1942. E. T. CASELLINI APPARATUS FOR COATING FLUORESCENTLAMP TUBING Filed Oct. 5, 1940 3 Sheets-Sheet 3 (kseZZz'rzz'.

IN VENTOR.

ATTOz/VEY Patented June 23, 1942 UNITED STATES PATENTQo F icE' Claims.

This invention relates to methods and means for coating the interiorsurface of glass bulbs or tubes and more particularly to the applicationhicles well-known.as nitrocellulosc solvents may beused to disperse thepowders. Those having a high solvent action,. such as acetone. onnitroof a luminescent coating on the interior walls of electric gaseousdischarge lamps or tubes.

An object of this invention .is to provide a method of applying auniform coating of fluorescent material to the inner walls ofja glassenvelope. I,

Another object is to provide a method of applying a coating offluorescent material to the inner walls of a fluorescent lamp or tubeinsuch a manner that the coating will have a uniform thickness at allpoints along the length of the bulb or tube.

A further object is to provide a method of applying a coating offluorescent material to the accompanying'drawings in which:

Figure I is a front perspective view of the tube coating and dryingapparatus I I Figure 2 is a front elevational viewpartly in section ofthe tube drying apparatus.

Figure 3 is a side elevational view partly in section of the tube dryingapparatus.

Figure 4 is a front elevational view partly in section of the tubecoating apparatus. v

Similar reference characters refer to similar parts throughout theseveral views of the-drawings.

One of the most difficult problemsin the process of applying a coatingof luminescent material to the inner walls of a glass envelope has beenthe obtaining of a smooth, even-lcoating free from streaks and of auniform thicknessat all points along the length of the glass envelope.

By employing the method of my invention, the

drying of the coating suspension on the walls of; the glass envelope iscontrolled in a manner as" to provide a smooth, even coating, free fromstreaks, mottling and uneven thickness alon the length of the envelope.

It has been found that a solvent such as acetone is desirable as thevehicle in which to suspend the luminescent material in the coating offluorescent lamps or tubing. Other organic veinner walls of afluorescent lamp or tube in such cellulose are desired .for they give alacquer of almost constant viscosity or consistency over a wide range ofnitrocellulose concentration. This means that as the lacquer dries,there will result only a slight difference in viscosity Poorer solventsuch as alcohol or amyl acetate may be used but then a more viscous flowof the material results as it dries. Viscous flow is not preferredbecause rapid drying will produce a finished coating showing runs and/orstreaks. A suspension having a low viscosity or consistency permits itto drain almost immediately from the tube and thus allow rapid drying ina short period of about a minute.

The air used to force the coating mixture up into the tubing is passedthrough an acetone saturation chamber before it enters the crock inwhich thecoating mixture is keptpBy thus causing the air tobecomesubstantially saturated with acetone before it comes in contact withthecoating mixture, t lieacetone in the mixture will not, as wouldotherwisehappen. vaporize under the influence of. the air and thuschange the powder concentration inthe coating mixture. It is desirablethat the powderto vehicle ratio in the coating mixture be kept withinpredetermined concentration limits, such as a specific gravity rangebetween 2.17 and 2.34 for the fluorescent. calcium tungstate phosphor ofabout .5 to 6 microns average with 28 microns maxi- .mum grain size inan acetone vehicle."

I suspend about;l4,000 grams of powder,

phosphor in a medium of 5500 cc. of acetone, 56

.. grains of one-half second viscosity nitrocellulose containing about30 per cent wetting alcohohand 195 cc. of dimethyl phthalate, the latterbeing added as a plasticizer to keep the dried nitrocellulose filmfrompeeling off the walls of the glass envelope when coated. l

Final adjustment for the operating specific gravity of the totalsuspension is accomplished by additions of acetone to obtain aspecific'gr'avity in the range between 2.17 and 2.34, for the phosphormentioned. 7

The amountof nitrocellulose may be'varied to obtain the desired coatingappearance. For example, if the amount of nitrocellulose be decreased byone-half, the above mixture-would produce a coating having a rough orsandy appearance which may be useful such as in covering glass defects.Larger amounts, as about per cent more, will give a coating having asatinlike appearance whereas much larger amounts will produce combmarks.

The above composition is largely determined by the particle size and thesurface character of the phosphor. For example, a smaller particlecondensation and diiiiculty in subsequent baking V out of the organicsin the coating. One-half the amount may be used satisfactorily. Theprimary 7 function of the dimethyl phthalate is to keep the lacqueredcoating on the tube from drying completely which may result in peelingdue to inflexible shrinkage of the film.

Other phosphors may be used.

A difierent formulation is necessary for the various phos- 1 phors ortheir blends. The criterion of their formulation is analogous to that ofthe calcium tungstate.

Since there is air present in the bulb or tube about to be coated, Ihave provided a saturation chamber connectedto the top of the tubing tobe'coated. Thus the air in the tubing willbe forced up into this chamberby the column of coating mixture. When this column reaches the top ofthe, tubing, the air pressure which forced it up into the tubing isreleased to the atmosphere and the column of coating mixture dropsrapidly back into the crock due to its low viscosity.

When the suspension in the tubing drops, air

length of the tubing mixture in the crock 3. The air which flows throughline I enters the saturation bell chamber 2 where it becomessubstantially saturated with acetone. This air then passes through line4 into the crock'3. Thecontrol of this air which forces the coatingmixture up line 6 and up into the tubing 8 is maintained through thehand pressure control It. Thus the operator can control the air,substantiallysaturated with acetone, which enters the crock 3' to forcethe coating mixture up into the tube 3 to the desired height.

Since there is a certain amount of air in the tubing before the coatingmixture is introduced therein, the coating mixture, as it moves .up thethe funnel l2 and line l3 and into the saturation chamber It. The natureof this chamber is such that it causes this air to become substantiallyThus, after the coating top of the tubing 3, the

saturated with acetone. mixture has reached the air pressure into thecrock 3 is released and the column of coating mixturein the tubing dropsback into the crock. Whenthe mixture drops,

' air will be drawn down into the tubing from the saturation chamber it.Since this air is substantially saturated with acetone, little or nodrying of the coating deposit, left on the walls of the tubing by theflushing, will take place, This condition is desirable, for drying isthereby prevented until the turbulent fiow of the coating has ceased anda smooth, uniform flow takes place. The length of time required fordrying is defluorescent lamp 30 mounted on the rack directlysubstantially saturated with acetone, will be drawn into the tubing fromthe saturation chamber. If this air, rushing into the tubing as thecolumn of coating mixture drops, were not-substantially saturated withacetone, the top part of the tubing would dryvery quickly due to theinrush of air on the quick-drying acetone and since the flow .of thecoating mixture would still be in turbulence, that is, an uneven,streamy flow,

thenits drying would resultin forming'a streaky or unevenly coatedtubing near the top. The

tubing is kept under this treatment until the flow along the wholetubing length has become sufficiently uniform so that the subsequentdrying will not produce drain marks or waves or streaks appearing in thedried coating.

In Figure 1, the air line I is connected to the saturation bell-chamber2 which is'in turn connected to the crock or container 3 through theline 4. This crock contains the luminescent coating mixture which. is tobe applied to the glass tubing. The motor I mounted on top of the crock3 has a shaft extending down through a substantially air-tightconnection into the coating mixture. A small propeller is attached tothe end of the shaft and is used to keep the coating mixturesulilciently agitated to prevent theluminescent powders from settlingout of their sus-' pension in the acetone. Line 3 extends from the lowersection of the crock 3 up through the trough I to the rubber stopperconnection in which the tube 3 to be coated is mounted. The exhaustpressure control 'lllare con- Hut and the hand nected to the air lineWhen the tubing 8 is to be coated, it is placed in the rubber stopperconnection I I, thus placing it in a position to be in contact with thecoating r the trough 1 behind the tubing or bytrial and is of the orderof about ten seconds. By keeping the air entering the tubingsubstantially saturated with acetone until most of the drainage hastaken place, the tubing has become sumciently drained to dry.

The tubing is then placed on the drying rack l5 as shown in Figures 2and 3. On the rim of is the vent 25 leading to the exhaust chamber whichis connected to the exhaust duct 32 for the purpose of drawing of! anyfumes from the coating mixture as it drains from the tube. Draining fromthe tube may be salvaged in the container 33 connected through line 34to the troughl. This rackl 5 consists of a frame on top of which thestatic mounted. through the air duct l1 and may be controlled by thedamper H. In order to prevent turbulence and venturi effects and toobtain a static prespressure chamber I3 is the tubes and resulting in auniform drying of the tube. If too much air is allowed to pass throughthe tubing being dried, the coating near the bottom of the tubing willbe too thickand if too littleair is used, thecoating near the bottomwill be too thin and may even flowout considerably. However, if it isfound that even when the coating is drying at about the same rate at allpoints along the entire length of the tubwill force the air up throughAir'is introduced into this chamber a static pressure chamber mountedthereon and from which a steady, evenly-diffused flow of air chambersmay take various forms and may be of dilferent types, for example abellows type instead of a bell chamber may be used. The bell chamber 2encases a series of concentrically wound wicks partially immersed inacetone. When the air passes through the chamber 2 from the air line Ito the line 4 leading into the crock 3 it becomes substantiallysaturated with acetone. There are three petcocks on this chamber,petcock 2| being for filling purposes, petcock 22 being for drainingpurposes and petcock 23 being used to gauge the height of the acetone inthe chamber. A glass visual gauge may be used instead of these petcocks.

The propeller 20 in the crock is very small in order to avoid thegeneration of heat in the coating mixture as a result of its motion.This propeller is used to keep the luminescent particles in suspension.It should turn in a direction such that the liquid is driven to thebottom of the crock, to mix again with the powder which has settled out.

The chamber l4 contains a quantity of acetone and a wick 24. The gauge26 acts as a means of determining the height of the acetone in thechamber. Although 1 have used this type of chamber, any chamber whichwill substantially saturate the air and be adaptable to my method may beused.

Although air has been used in forcing the coating mixture up into thetube to be coated, other suitable gases may readily be used. Althoughthe exhaust duct 32 connected to the vent 25 in the trough l is used todraw off fumes from the coating, it has been found that since,

these fumes and gases are heavier than air, this exhaust arrangement maybe used to obtain the proper drying of the coating on the tube.

What I claim is:

1. Apparatus for coating the inner surface of glass tubing with aluminescent coating mixture said apparatus comprising: a source of airat high pressure; a saturation chamber connected to said source of air;a container for the luminescent coating mixture, said container beingconnected to said saturation chamber; a second saturation chamber inregister with the top of the tube being coated; and a drying rack havingis caused to be directed down into the tube to dry the coating ofluminescent material therein.

2. Apparatus for coating the inner surface of glass tubing with aluminescent coating mixture, said apparatus comprising: a closedcontainer for the luminescent coating mixture, said container having aninlet for air and an outlet for said mixture; a holder to keep the tubebeing coated in contact at its bottom with said outlet; and a saturationchamber in position to fit over the top of said tubing being coated.

3.'Apparatus for coating the inner surface of glass tubingwith aluminescent coating mixture, said apparatus comprising: a saturationchamber; a closed container for the luminescent coating mixture, saidcontainer having an inlet for air and an outlet for said mixture, saidinlet for air being connected to said saturation chamber; a support tohold the tubing to be coated in register with said outlet; and a secondsaturation chamber in position to fit over the top of the tubing to becoated.

4. Apparatus for coating the inner surface of glass tubing with aluminescent mixture, said apparatus comprising: a saturation chamber; aclosed container for the luminescent coating mixture, said containerhaving an inlet for air and an outlet for said mixture, said inlet forair being connected to said saturation chamber; a

support to hold the tubing to be coated in register with said inlet; asecond saturation chamber in position to fit over the top of the tube tobe' coated; and a drying rack comprising a static pressure chambermounted thereon and from which a steady, evenly-diffused flow of air iscaused to be directed down into the tube to dry the coating ofluminescent material therein.

5. Apparatus for coating the inner surfaceof glass tubing with aluminescent coating mixture, said apparatus comprising: a closedcontainer for the luminescent coating mixture, said container having ainlet forair and an outlet for said mixture; a sa uration chamberconnected to said inlet for air; and a support for tubing to be coated,in register with said outlet.

' EZIO THOMAS CASELLINI.

